Saturday, May 16, 2009

Link utilization

One of the basic tasks in monitoring network traffic is to accurately track the utilization of links in your network. A managed switch will provide a standard set of counters for each interface that can be retrieved retrieved periodically using SNMP and used to trend link utilization, packet rates, errors and discards.

sFlow provides an alternative to SNMP counter polling. The sFlow agent in the switch will periodically send, or "push" its own counters to the central collector. Pushing counters is much more efficient than than retrieving them using SNMP, requiring 10-20 times fewer network packets to retrieve the same information. The sFlow protocol uses XDR to encode the counters. XDR is much simpler to encode and decode than the ASN1 encoding that the SNMP protocol uses, so the CPU load on the switches and the collectors is also significantly reduced. Finally, distributing the counter polling task among the switches further reduces the load on the central collector.

The benefits of using sFlow to retrieve interface statistics become clear when you monitor large networks. Instead of requiring 5-10 servers dedicated to SNMP polling, a single sFlow analyzer can collect counters from all the interfaces in the network, providing a centralized view of utilization throughout the network, rapidly identifying any areas of congestion.

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